There's a change machine in my local supermarket. Drop in a pound's worth of coppers – the irritating one and two pence pieces that just weigh your pocket down and that you hardly ever spend – and you get a shiny £1 coin back.
It's a great idea, and one that seems to have been ripped off by Perry Noble's NewSpring church in South Carolina.
The megachurch pastor has a reputation for controversy, and it'll be interesting to see how many eyebrows his current project raises. Because the church is encouraging its members to tithe – give 10 per cent of their income – by offering a 90-day money back guarantee.
That's right: "Because we believe what the Bible says about tithing, we commit to you that if you tithe for three months and God doesn't hold true to His promises of blessings (Malachi 3:10), we will refund 100% of your tithe. No questions asked. "
In a video clip, Noble says, again citing Malachi: "If we will put God first with the tithe, He will bless us, in fact, He will open up the flood gates of Heaven and pour out so much blessing on us that we won't have room enough for it." And, "in 90 days, if you don't feel like God has blessed you, if you don't feel like God has done what His word has said, if you believe God's a liar here's what we'll do. We'll refund every dime you gave during that 90-day period."
Links to articles explaining more on NewSpring's page weren't working when I clicked on them, but the titles and subtitles give an indication of what they say. "Why you can't afford not to tithe: Once you've seen God's provision in tangible ways, it's easy to see that you can't afford to not..." "6 questions about tithing answered: It takes a leap of faith to write that first tithe check. There are bills to be paid, expenses you haven't thought..."
And there's a video interview with church member Roshanda Fuller, who gave what she thought was her last $11 and found she still had enough to meet her needs. While the blessing "might not be monetary", she says, she's learned to trust God with her finances.
I'm genuinely happy for her, and for others who've found tithing works for them.
But God is not a slot machine.
You do not approach him with one hand held out to give and the other held out to get. It doesn't work that way.
Many Christians tithe. It's not a bad thing to do, and we're certainly called to live generously. It's useful for churches, which always need money, but it isn't a New Testament principle enjoined on Christians. Malachi 3:10 ("Bring the whole tithe into the storehouse, that there may be food in my house. Test me in this,' says the Lord Almighty, 'and see if I will not throw open the floodgates of heaven and pour out so much blessing that there will not be room enough to store it,'") was written, in all probability, during the rule of Nehemiah and the restoration of the Temple. It says that if the people of God are faithful to him, he will bless them. It cannot possibly be used to argue that Christians should tithe, and that if they do they won't miss it because God will give it back to them.
Tithing is a good benchmark, if you can afford it. But not everyone can afford it, no matter what NewSpring says. Children need to be fed and clothed, rent needs to be paid. People have responsibilities.
In Mark 7: 11-12 Jesus condemns those who deny their family's needs because they put their religious duties first, saying: "Thus you nullify the word of God by your tradition that you have handed down."
Let's have some honesty about giving.
If you give, you give – you don't expect that money back. You expect to be poorer. If you give sacrificially, you accept your life will be more limited than it might have been. You won't go on such exotic holidays. Your phone may not be the latest, you won't eat out so much, your car might be rusty.
Most of the time, you don't mind, because God is worth it. Sometimes you do, because you're human.
But giving in to the belief that you and God have a sort of contract, whereby he's obliged to make up any losses incurred in his service, is so far wrong that it's verging on heresy. It says nothing about grace, love or freedom. It makes discipleship into a commercial transaction.
If you want to tithe, tithe. But it'll cost you, so make sure you can afford it.
Follow Mark Woods on Twitter: @RevMarkWoods